The invention relates to a manufacturing process for paper, the word "paper" denoting, in the following any flat structure or sheet not only based on cellulose fibers--the most frequently raw material used in the paper and cardboard industry--but also on the basis of
synthetic fibers such as polyamide, polyester and polyacrylic resin fibers, PA1 mineral or inorganic fibers such as asbestos, ceramic and glass fibers, PA1 any combination of cellulosic, synthetic and inorganic fibers. PA1 R.sub.1 and R.sub.2 each represent, independently of one another a straight or branched chain alkyl radical of C.sub.1 -C.sub.4 or are joined in a ring structure, PA1 R.sub.3 represents an alkyl radical with a straight or branched chain of C.sub.1 -C.sub.4 and n represents a whole number from 1 to 3. The cationisation reagents used are preferably: PA1 diethylamino chloroethane, PA1 trimethylammonium epoxypropyl chloride, PA1 trimethylammonium chloro-1-hydroxy-2-propane chloride. PA1 in the case of starch phosphonates, aminochlorethane diethylphosphonic acid, PA1 in the case of starch sulfates, sulfamic acid, sulfamates or again electron donor SO.sub.3.sup.- complexes such as SO.sub.3 -TMA (trimethylamine), SO.sub.3 -pyridine, PA1 in the case of starch sulfoalkyls, 2-chlorethane-sulfonates and 3-chloro-2-hydroxypropanesulfonate, PA1 in the case of starch carboxyalkyls, salts of 1-halocarboxylic acids such as sodium monochloracetate or sodium chloropropionate, lactones like propiono- or butyrolactone, acrylonitrile (reaction followed by a saponification), acid anhydrides such as maleic, succinic, phtalic anhydrides and the like, PA1 in the case of starch sulfocarboxyls, 3-chloro-2-sulfopropionic acid. PA1 "Starch: Chemistry and Technology", edited by Whistler et al, vol. II (Industrial aspects), 1967, Academic Press; PA1 "Starch Production Technology", edited by J. A. Radley, 1976, Applied Science Publishers Ltd. London; PA1 "Starch: Chemistry and Technology", edited by Whistler et al, 2nd edition (1984), Academic Press, Inc., pages 354-385].
The well-known use of cationic starches which are introduced into the fiber mass before the formation of the sheet, has permitted increase in the retention of the fibers and of the fillers, improvement in the draining and increase in the physical characteristics of the paper; in fact, the preferential fixation of these starches to the anionic reaction sites of the fibers and of the fillers, made possible by their cationic character or cationicity, enables the number of bonds between fibers to be increased as well as between fibers and fillers, whence a greater strength of the paper; and due to this greater strength of the paper, it became possible to reduce the concentration of the fiber mass or to resort to lower quality fibers.
Now, the advantages procured by the employment of cationic starches do not always permit, since a couple of years, compensation of the increasing drawbacks created by the increasing degradation in the quality of starting or raw materials.
In fact, to face up to stricter and stricter concerns of economic profitability, not only the semichemical pulp traditionally used, for example, for the manufacture of the paper for corrugated cardboard has seen its part reduced to the profit of pulps derived from recovered cellulose fiber, commonly called RCF, but in addition the quality itself of this RCF is more and more mediocre by reason of the increasing number of recyclings of "old paper".
To this is added the fact that at the level of paper machines, the tendency is more and more towards the systematic closing of circuits, whence an enrichment of the manufacturing liquors with organic and inorganic substances.
These factors cooperate to the reduction in the solidity of the paper; the proportion of cationic starches which can be fixed to the fibers decreases, whence a lower strength of this sheet.
Various solutions have been proposed to overcome these drawbacks.
Thus, starches have been developed characterized by a cationicity which is higher and higher, but which is quite obviously limited by the maximum cationicity that can be achieved by conventional processes of producing cationic starches. And in any case, whatever the degree of cationicity, the closure of circuits and the degradation of the quality of the fibers is manifested by an ineluctable lowering in the strength of the papers.
Knowing that the effectiveness of a cationic starch is all the greater as its probability of fixation of the fiber is greater, recourse has been had (see U.S. Pat. No. 4,066,495) to increase this probability of fixation, to associations of the "cationic starch--polyacrylamide" or "cationic starch--aluminum sulfate or aluminum polychloride" type.
This use of two or several compounds of the same ionicity has only in fact the sole object of increasing the retention of the fillers and of the fibers without however modifying the composition of the paper.
Within the same order of ideas, starches have been resorted to (see Patent FR 1,499,781) containing both cationic groups and anionic phosphate groups.
These starches, although comprising consequently, groups of different ionicity, have in spite of all, an essentially cationic character, involving consequently their own limits of utilisation.
The successive application of a starch phosphate and of a cationic starch only permits improvement in the strength of the paper obtained and this in an insufficient proportion. In addition, these phosphated starches contribute to increasing the polluting charge through the presence of nitrogen compounds evolved in their manufacturing process.
In so called "dual" techniques, it is not to starches comprising both cationic groups and phosphate groups or to the employment of starch phosphates and of cationic starches that recourse is had, but to associations of cationic starches that recourse is had, but to associations of cationic starches and compounds of different ionicity.
Thus (see EP 41,056) cationic starches have been used in association with colloidal silicic acid; besides, the patent EP 60,291 discloses the preparation of a gel based on cationic starch and carboxymethylcellulose or of an uronic acid polymer, this gel being partly dehydrated by the action of a colloidal solution of polysilicic acid or of an oxypolyaluminum compound.
The dual techniques lead to an improvement in retention, thus permitting the manufacture of a paper with a higher content of fillers. They permit a substantial economy in cellulose, but are not applicable in all cases. Moreover, the amount of starch fixed to the cellulose at the time of formation of the sheet remaining still limited, the physical characteristics of the paper so obtained are not always improved sufficiently.
To obtain a paper having increased mechanical characteristics, it is consequently necessary to subject the paper produced according to one of the preceding techniques, to a surface treatment carried out, particularly in a machine of the "size-press" type; such a treatment permits the proportion of starch entering into the constitution of this paper to be increased, thus conferring on it a better strength.
Now such a solution is not satisfactory from the economic point of view, any additional operation being expensive; passage in a "size-press" results besides in a considerable reduction, of the order of 15 to 20%, in the speed of the machines and hence in paper production.
It results from the foregoing that none of the existing processes leads, at a satisfactory cost price, to the production of paper having the desired characteristics.
It is therefore a particular object of the invention to provide a paper manufacturing process responding better to the various desiderata of practice than those existing hitherto.